1. Field of the Invention
The present invention relates generally to telephony networks, and in particular, to a telecommunications system that allows isochronous networks to be interconnected by non-isochronous packet-switched networks.
2. Description of the Related Art
A digital communications network simultaneously connects many users to one another over shared transmission paths. To accomplish this, these networks typically include some form of xe2x80x9cswitchingxe2x80x9d. Switching enables the reconfiguration of point-to-point connections in the network, and usually takes one of two basic formsxe2x80x94circuit switching or packet switching.
In circuit switching, the network connects a bit stream to a destination for relatively long periods of time using a dedicated circuit. For example, in a telecommunications network, the circuit is formed for the duration of one telephone call. An important property of circuit switching is the need to set up an end-to-end path before any data can be sent over the network. Once a call has been set up, the dedicated circuit between nodes exists, and will continue to exist until the telephone call is finished. As a consequence of establishing a dedicated circuit, there is no danger of having the call disconnected due to excessive network traffic.
An alternative to circuit switching is packet switching. When this form of switching is used, no dedicated path is established in advance between the transmitter and receiver. Instead, when the transmitter is ready to send data, the data is encapsulated into relatively short bundles of bits called xe2x80x9cpacketsxe2x80x9d, and a destination address is appended. Then, the packet is sent over the network to the destination address without using a dedicated circuit. The advantage of packet switching is that the bit stream between the source and destination includes only the packets needed to transmit data. This leads to a more efficient use of network bandwidth.
The key difference between packet-switching and circuit-switching is that circuit switching statically reserves the required bandwidth in advance, whereas packet switching acquires and releases it as it is needed. With circuit switching, any unused bandwidth on an allocated circuit is wasted. In contrast, with packet switching, unused bandwidth can be utilized by other packets from unrelated sources because the transmission circuits are not dedicated to individual sources.
Telecommunication devices are typically interconnected using circuit-switched networks, such as the T1, T2, T3, and T4 carriers developed by the Bell System, or the E1, E2, E3 carriers established by CCITT. These standard networks are isochronous, that is, data is transferred between devices which are operating at the same average bit rate.
In addition to being isochronous, conventional telecommunications networks are designed to handle pulse-coded modulated (PCM) voice as well as signalling information. PCM voice is a digital representation of voice input or computer data that is transmitted through a conventional analog telephone input using a modem. Signalling information is digital data that does not represent PCM voice and is generally used for call setup.
FIG. 1 illustrates a prior art subscriber telecommunications network. The network include a common control shelf 10 connected to two remote peripheral shelves 12 using a conventional circuit-switched network 14. The common control shelf 10 is also connected to the central office (CO) 16 of a common carrier using a public switched telephone network (PSTN) 17. The circuit-switched network 14 and the PSTN 17 are both isochronous networks. One or more peripheral units 15, such as telephones, modems, faxes, etc., are connected to the peripheral shelves 12.
The common control shelf 10 mediates digital communications among the peripheral shelves 12, as well as communications between the peripheral shelves 12 and the CO 16. The common control shelf 10 provides switching and call setup services to the peripheral shelves 12 and can be implemented using a private branch exchange (PBX), such as the Hicom 300 E manufactured by Siemens Corporation.
Each peripheral shelf 12 includes a plurality of analog or digital ports and interface circuits for connecting a group of peripheral units 15. All calls placed between peripheral units or to the CO 16 are routed through the common control shelf 10. The peripheral shelves 12 can be local line trunk units (LTUs) or remote LTUs located several kilometers (up to 700 miles) from the common control shelf 10. Remote LTUs can be implemented using a Remote Communication Module (RCM) manufactured by Siemens Corporation. Private subscriber networks, such as shown in FIG. 1, can be used to interconnect users within a local campus. For example, in a campus environment having many distributed users, RCMs can be remotely located throughout the campus to serve various groups of telecom users. The RCMs can be connected to one or more PBXs using standard circuit-switched network technology. Typically, a PBX is used to interconnect a plurality of RCMs or LTUs allowing calls to be placed between various groups located within the campus. In addition, the PBX can also be attached to a telephone company""s central office, allowing users to place calls outside the campus. A PBX typically provides bandwidth and switching capacity to the LTUs/RCMs and thus, a PBX can act as a central switching hub for several RCMs and LTUs.
Although PBXs provide communications bandwidth, they can only support a limited number of users. When new users are added to the campus system, it is often necessary to increase the telecommunications bandwidth by adding more LTUs and RCMs or more PBXs. In many instances, this is very expensive because adding new PBXs and LTUs and RCMs requires additional copper cabling and networking of the PBXs.
In other situations, it is often necessary to move users to different locations within a campus. This frequently requires additional copper cabling. Thus, a simple move can become very expensive because of the high costs of relocating conventional phone lines and PBXs.
In order to eliminate the cost of copper cabling it is known to provide circuit-switched PBXs and RCMs that rely on fiber optic technology. However, these PBXs and RCMs are generally expensive. Moreover, incorporating fiber-optic telecommunications equipment into pre-existing copper-based subscriber networks typically requires costly upgrades that are often unacceptable to many customers.
It is also known to use low-cost packet networks for telephony. For instance, U.S. Pat. No. 5,594,732 discloses a network bridge for transferring isochronous signalling messages using a non-isochronous packet network. However, this reference does not disclose a device that performs the multiplexing/demultiplexing associated with isochronous telephony, as well as the xe2x80x9cpacketizingxe2x80x9d associated with packet-switched networks. Therefore, the ""732 patent does not provide a device that can be easily integrated into an existing telephone network.
Therefore, there is a need for a system and method that allows a conventional telephony network to be expanded at a lower cost and without the need for replacing the existing infrastructure of switching gear.
It is an advantage of the present invention to overcome the limitations of the prior art telecommunications networks. It is also an advantage of the present invention to provide a system and method that allows a low-cost packet-switched network to be inserted into an isochronous circuit-switched network. This allows public or private telecommunications networks to be upgraded and expanded with a substantial savings in cost.
These and other advantages of the present invention are realized by providing a novel system and method for transmitting isochronous telecommunications data frames over a packet-switched network.
According to one embodiment of the present invention, a telecommunications system is provided that includes a first controller and a second controller coupled using a packet-switched network. The first controller receives one or more isochronous input channels from a common control shelf, such as a private branch exchange (PBX), over a circuit-switched network. The first controller encapsulates an isochronous frames from the common control shelf into a data frame that is transferrable over the packet-switched network. The second controller is configured to receive the data frame and extract the isochronous frame, which is then transmitted to a peripheral shelf, such as a line trunk unit (LTU), using an isochronous circuit-switched network.
The controllers contain identical functionality, permitting bi-directional transmission of data between the common control and peripheral shelves. Each controller includes multiplex/de-multiplex functions normally associated with isochronous telephony, as well as a packetize/de-packetize functions for transferring data over the packet-switched network.